Shankhadeep Chakraborty

RISING WATER SALINITY: A THREAT TO MANGROVES OF INDIAN SUNDARBANS

Abstract Mangroves constitute an important ecosystem because of their global extent and high productivity. These plants thrive in the intertidal zones of the tropics and subtropics that are characterized by regular tidal inundation and fluctuating salinity. Mangrove species are well adapted, both morphologically and physiologically, to survive under saline conditions, but in hypersaline environment their growth is reduced. The present chapter is a critical analysis on the impact of salinity on the growth of a common mangrove species (Hertiera fomes). The analysis has been carried out in the framework of Indian Sundarbans, which has contrasting salinity profiles in different segments owing to barrage discharge and siltation phenomena. Analysis of the decadal profile of salinity indicates a gradual lowering in the western Indian Sundarbans due to Farrakka barrage discharge and run-off from catchments. The central sector, however, exhibits a contrasting picture of increment of aquatic salinity through time, mainly due to disconnection of the Bidyadhari River with the Ganga–Bhagrirathi–Hooghly River system (in the western part). This has made the Matla River in the central Indian Sundarbans hypersaline in nature (that used to get water from the Bidyadhari River) finally leading to an insecure ecological condition for the growth and survival of mangroves. The possible remedial measures to combat the situation have also been listed considering the ecological framework of the study zone.

Decadal Variation of Nutrient Level in Two Major Estuaries in Indian Sundarbans

The impact of the nutrient level on the water quality in the Hooghly and Matla estuarine complex in Indian Sundarbans was assessed for three decades (1984-2014). Nitrate, phosphate and silicate were used as indicators of nutrient related water quality in the estuarine water. Our first order analysis reflects significant spatio-temporal variations of selected nutrients with relatively higher values in the Hooghly estuary (in the western Indian Sundarbans) compared to the Matla estuary (in the central Indian Sundarbans). Significant variations were observed in dissolved nitrate, phosphate and silicate concentrations between stations and years (p < 0.01). Such pronounced variations may be attributed to the location of highly industrialized and urbanized city of Kolkata, Howrah and Haldia port-cum-industrial complex adjacent to the Hooghly estuary. The sudden rise of selected nutrients during premonsoon, 2009 (irrespective of sampling stations) is directly related to AILA, a super-cyclone that occurred in the lower Gangetic delta during 22nd 25th May, 2009.

Accumulation of Metals by Mangrove Species and Potential for Bioremediation

The Indian Sundarbans region at the apex of the Bay of Bengal in Eastern India is impacted by industrial and urban development, including the discharge of large volumes of wastewater. We analyzed the concentrations of zinc (Zn), copper (Cu) and lead (Pb) in the root, stem and leaves of four dominant mangrove species, Avicennia officinalis, Avicennia marina, Avicennia alba and Excoecaria agallocha in samples collected from 12 different monitoring stations in the Indian Sundarbans. At all of the stations, the magnitude of accumulation of the metals in the vegetative parts of the mangroves was in the order, root > stem > leaf. The maximum concentrations of zinc in the roots (116.1 ± 4.2 μg/g dry weight) and the stems (100.4 ± 6.2 μg/g dry weight) was observed in A. officinalis collected at the Kakdwip site, whereas in leaf tissues, the maximum mean zinc concentration (66.8 ± 3.5 μg/g dry weight) was observed in A. marina collected at the Chemaguri site. The concentrations of copper were lower, with the maximum mean concentration (51.1 ± 3.0 μg/g dry weight) in the roots of A. officinalis at Kakdwip. In all vegetative parts, the concentrations of lead were lowest relative to the other metals, with the maximum mean concentration (30.4 ± 2.0 μg/g dry weight) observed in the roots of A. officinalis at Kakdwip. The concentrations of metals were generally lower in the vegetative parts of E. agallocha relative to the other mangrove species. Simultaneous analysis of dissolved metals in the surface waters at the monitoring stations revealed that zinc was present in high concentration, followed by copper and lead. Among the 12 selected stations, the highest concentrations of dissolved zinc, copper and lead were observed at Kakdwip at concentrations of 668.5 ± 5.4, 176.3 ± 4.6 and 48.4 ± 3.1 μg/L, respectively. This site is located in the western part of the Indian Sundarbans. The site at Bagmara in the eastern part of the study area had the lowest concentrations of dissolved heavy metals, as well as the lowest concentrations of the metals in sediments. High metal concentrations in sediments observed at the Frasergunj site (117.3 ± 4.3 μg/g dry weight for zinc, 37.7 ± 2.4 μg/g dry weight for copper and 10.8 ± 1.0 μg/g dry weight for lead) may be due to the presence of a fish landing station at this site. There were significant correlations between the levels of the metals in ambient media (i.e. water, sediments) and the concentrations of the metals in the vegetative parts of the mangroves; indicating that the levels of metals in mangroves reflect environmental exposures. The results indicate that mangrove species can accumulate these metals from the marine environment to ppm concentrations. Bioaccumulation factors for A. officinalis vegetation relative to sediment appeared to be >1 for lead, which is discussed in the context of using this species for bioremediation; a new dimension in the spectrum of ecosystem services offered by the mangroves.

Mangrove Health Card: A Case Study on Indian Sundarbans

Relative abundance and Shannon-Weiner species diversity index of true mangrove species were estimated in 10 sampling stations of Indian Sundarbans during 2013. The mangrove patches in the selected stations were categorized into a 4-point scale depending on the values of Shannon-Weiner species diversity index. The health of the mangrove forest as per our constructed 4-point scale was found to be regulated primarily by anthropogenic factors, although in station like Sagar South, the natural erosion may be the key player in determining the mangrove floral diversity.